1. Field of the Disclosure
Embodiments disclosed herein relate generally to screen separators, and in particular to vibrating screen separators.
2. Background
Oilfield drilling fluid, often called “mud,” serves multiple purposes in the industry. Among its many functions, the drilling mud acts as a lubricant to cool rotary drill bits and facilitate faster cutting rates. Typically, the mud is mixed at the surface and pumped downhole at high pressure to the drill bit through a bore of the drillstring. Once the mud reaches the drill bit, it exits through various nozzles and ports where it lubricates and cools the drill bit. After exiting through the nozzles, the “spent” fluid returns to the surface through an annulus formed between the drillstring and the drilled wellbore.
Furthermore, drilling mud provides a column of hydrostatic pressure, or head, to prevent “blow out” of the well being drilled. This hydrostatic pressure offsets formation pressures thereby preventing fluids from blowing out if pressurized deposits in the formation are breeched. Two factors contributing to the hydrostatic pressure of the drilling mud column are the height (or depth) of the column (i.e., the vertical distance from the surface to the bottom of the wellbore) itself and the density (or its inverse, specific gravity) of the fluid used. Depending on the type and construction of the formation to be drilled, various weighting and lubrication agents are mixed into the drilling mud to obtain the right mixture. Typically, drilling mud weight is reported in “pounds,” short for pounds per gallon. Generally, increasing the amount of weighting agent solute dissolved in the mud base will create a heavier drilling mud. Drilling mud that is too light may not protect the formation from blow outs, and drilling mud that is too heavy may over invade the formation. Therefore, much time and consideration is spent to ensure the mud mixture is optimal. Because the mud evaluation and mixture process is time consuming and expensive, drillers and service companies prefer to reclaim the returned drilling mud and recycle it for continued use.
Another significant purpose of the drilling mud is to carry the cuttings away from the drill bit at the bottom of the borehole to the surface. As a drill bit pulverizes or scrapes the rock formation at the bottom of the borehole, small pieces of solid material are left behind. The drilling fluid exiting the nozzles at the bit acts to stir-up and carry the solid particles of rock and formation to the surface within the annulus between the drillstring and the borehole. Therefore, the fluid exiting the borehole from the annulus is a slurry of formation cuttings in drilling mud. Before the mud can be recycled and re-pumped down through nozzles of the drill bit, the cutting particulates must be removed.
Apparatus in use today to remove cuttings and other solid particulates from drilling fluid are commonly referred to in the industry as “shale shakers.” A shale shaker, also known as a vibratory separator, is a vibrating sieve-like table upon which returning solids laden drilling fluid is deposited and through which clean drilling fluid emerges. Typically, the shale shaker is an angled table with a generally perforated filter screen bottom. Returning drilling fluid is deposited at the feed end of the shale shaker. As the drilling fluid travels down length of the vibrating table, the fluid falls through the perforations to a reservoir below leaving the solid particulate material behind. The vibrating action of the shale shaker table conveys solid particles left behind until they fall off the discharge end of the shaker table. The above described apparatus is illustrative of one type of shale shaker known to those of ordinary skill in the art. In alternate shale shakers, the top edge of the shaker may be relatively closer to the ground than the lower end. In such shale shakers, the angle of inclination may require the movement of particulates in a generally upward direction. In still other shale shakers, the table may not be angled, thus the vibrating action of the shaker alone may enable particle/fluid separation. Regardless, table inclination and/or design variations of existing shale shakers should not be considered a limitation of the present disclosure.
Preferably, the amount of vibration and the angle of inclination of the shale shaker table are adjustable to accommodate various drilling fluid flow rates and particulate percentages in the drilling fluid. After the fluid passes through the perforated bottom of the shale shaker, it can either return to service in the borehole immediately, be stored for measurement and evaluation, or pass through an additional piece of equipment (e.g., a drying shaker, centrifuge, or a smaller sized shale shaker) to farther remove smaller cuttings.
A plurality of motions has been commonly used for the screening of materials, including linear, round, and elliptical motion. Currently, when a drilling operator chooses a separatory profile, therein selecting a type of motion that actuators of the vibratory separator will provide to the screen assemblies, they typically choose between a profile that either processes drilling material quickly or thoroughly. It is well known in the art that providing linear motion increases the G-forces acting on the drilling material, thereby increasing the speed of conveyance and enabling the vibratory separator to process heavier solids loads. By increasing the speed of conveyance, linear motion vibratory shakers provide increased shaker fluid capacity and increased processing volume. However, in certain separatory operations, the weight of solids may still restrict the speed that linear motion separation provides. Additionally, while increased G-forces enable faster conveyance, as the speed of conveyance increases, there is a potential that the produced drill cuttings may still be saturated in drilling fluid.
Alternatively, a drilling operator may select a vibratory profile that imparts lower force vibrations onto the drilling material, thereby resulting in drier cuttings and increased drilling fluid recovery. However, such lower force vibrations generally slow drilling material processing, thereby increasing the time and cost associated with processing drilling material.
Round motion may be generated by a simple eccentric weight located roughly at the center of gravity of a resiliently mounted screening device with the rotational axis extending perpendicular to the vertical symmetrical plane of the separator. Such motion is considered to be excellent for particle separation and excellent for screen life. It requires a very simple mechanism, a single rotationally driven eccentric weight. However, round motion acts as a very poor conveyor of material and becomes disadvantageous in continuous feed systems where the oversized material is to be continuously removed from the screen surface. Machines are also known with two parallel axes of eccentric rotation extending perpendicular to the symmetrical plane.
Another common motion is achieved through the counter rotation of adjacent eccentric vibrators also affixed to a resiliently mounted screening structure. Through the orientation of the eccentric vibrators at an angle to the screening plane, linear vibration may be achieved at an angle to the screen plane. Such inclined linear motion has been found to be excellent for purposes of conveying material across the screen surface. However, it has been found to be relatively poor for purposes of separation and is very hard on the screens.
Another motion commonly known as multi-direction elliptical motion is induced where a single rotary eccentric vibrator is located at a distance from the center of gravity of the screening device. This generates elliptical motions in the screening device. However, the elliptical motion of any element of the screen has a long axis passing through the axis of the rotary eccentric vibrator. Thus, the motion varies across the screening plane in terms of direction. This motion has been found to produce efficient separation with good screen life. As only one eccentric is employed, the motion is simple to generate. However, such motion is very poor as a conveyor.
U.S. Pat. No. 6,513,664 discloses a vibrating screen separator that may be operated in linear or elliptical modes. The shaker allows operators to use linear motion while drilling top-hole sections where heavy, high-volume solids such as gumbo are usually encountered. In these intervals, shakers need to generate high G-forces to effectively move dense solids across the screens. As conditions change, the shaker can be adjusted from linear to balanced elliptical motion without shutting down the shaker. Operating in the gentler balanced elliptical mode, solids encounter reduced G-forces and longer screen residence time.
Another motion similar to the counter rotation of adjacent eccentric vibrators is illustrated in U.S. Pat. No. 5,265,730. Uni-directional elliptical motion is generated through the placement of two rotary eccentric vibrators with the axes of the vibrators similarly inclined from the vertical away from the direction of material travel and oppositely inclined from the vertical in a plane perpendicular to the direction of material travel. The inclination of the large axis of the elliptical motion relative to the screen surface is controlled by the inclination of the rotary eccentric vibrators away from the intended direction of travel of the material on the screen surface. The inclination of the vibrators in a plane perpendicular to the intended direction of material travel varies the width of the ellipse. These devices have been found to require substantial frame structures to accommodate the opposed forces imposed upon the frame.
In general, the efficiency of the shaker may be influenced by the vibration pattern of the shaker, as described above. The vibratory motions described above are typically imparted to the shaker screen through rotation of at least one unbalanced weight by a rotary motor. Shaker efficiency may also be influenced by the vibration dynamics, or G-force imparted to the particles due to the shaking. Other variables that may influence efficiency include deck size and configuration, shaker processing efficiency, and shaker screen characteristics. The angle of the shaker screen, or deck angle, relative to horizontal may also affect separation efficiency. Deck angle is often controlled hydraulically, and can be automated or manually adjusted.
As described above, control of the vibratory pattern of the shaker, deck angle, and other variables may affect shaker efficiency. Additionally, multiple component parts are used to independently control these variables. The vibrations of the vibrating screen separator, in addition to normal wear and tear, subject these component parts to fatigue and failure.
Accordingly, there exists a need for a shaker having improved control of shaker variables, fewer component parts, and/or improved motion control.